Although HIV-1 integrase remains a priority target for development of small-molecule antagonists, a high-resolution structure of the intact molecule remains elusive. Using a combination of protein footprinting and mass spectrometry, we have been successful in defining the binding site for pyridoxal phosphate on intact HIV-1 integrase. NMR studies with the polypurine tract primer of (+) strand DNA synthesis have demonstrated a change in sugar ring conformation at the PPT-U3 junction, suggesting this may be an important determinant for its recognition by the RNase H domain of HIV-1 reverse transcriptase (RT). Finally, targeted insertion of nucleoside analogs has defined regions of the nucleic acid substrate that interact with the DNA polymerase domain of Ty3 RT. An extension of this study investigated whether Ty3 RT mutants were capable of reversing analog-induced inhibition of DNA synthesis. Such biochemical complementation highlighted residues of the Ty3 thumb subdomain that contact individual bases of the template-primer duplex, illustrating the important of nucleoside analog interference strategies. [Corresponds to Le Grice Project 2 in the April 2007 site visit report of the HIV Drug Resistance Program]